The user perceived quality is based on the receiving capabilities of the users (number of layers) and the quality properties of the layered media. The layered media to be multicast typically consists of one base layer and some enhancement layers each providing progressive quality improvement. There are available codec's (like Advanced Audio Coding—Bit-Sliced Arithmetic Coding (AAC-BSAC) for audio and H.264/MPEG-4 AVC for video) providing such a media stream.
In the wireless access network, different data rates (e.g. provided by different modulations) are available for the Access Point (AP) and the AP can change the applied data rate per packet. For example WLAN 802.11b networks have this feature. However, this feature is not used in case of multicasting.
From the adaptiveness point of view three multicasting schemes are used today:                Single stream adaptive approach: the highest possible quality, but the same quality is sent to all users according to their receiving capabilities.        Replicated stream approach: different quality versions of the media are sent to the users according to their receiving capabilities.        Layered stream approach: different layers of the media are sent out to different multicast groups, the user can decide to which group he/she wants to sign up.        
All the above approaches are available today in wireline networks. However, only the Single stream adaptive approach is used in wireless networks. In the typical realization, the media is transmitted at a fixed data rate, mostly at the lowest. The realization of this approach is referred to as Single Rate later on.
The Replicated and the Layered approaches require additional information like the receiving capability of the users. However, this capability information is not used today in case of multicasting if it is available at all. In WLAN networks, for example, there is no protocol to report this information. A possible realization of the Replicated approach is referred to as Replicated Rate later on.
In the Layered approach, the data packets have to carry information about which layer they correspond to. The solution used today in wireline environment is to distribute each layer on a separate multicast group, however, it cannot be applied in wireless networks, since the continuously changing radio environment would cause continuous subscription and leave of these groups. An applicable solution can be to provide the layer information, for example, by cross-layer information forwarding techniques like the LID concept, using the Differentiated Services CodePoint, or other appropriate fields in packet headers. In that case, all layers of a media can be distributed in the same multicast group.
Moreover, in order to utilize the advantages of the Layered approach and provide better solutions than the Replicated approach, the available data rates should be nested. Namely, if a higher data rate is available in an area, then all lower data rates are also available. See FIG. 1 for illustration in case of WLAN 802.11b networks. Depending on range from the access point different bit rates can be applied and received in the radio interface. The further away from the AP the receiver is the less the bit rate will generally be. Many WLAN systems have a solution for decreasing the bit rate using a scheduled process until a suitable bit rate is established for a specific user. In FIG. 1 four different bit rate levels are indicated: 1, 2, 5.5 and 11 Mbps (Mega Bit per Second).
The above listed requirements set by the Layered approach are not satisfied today in wireless networks and the inherent capabilities to optimize the wireless multicast for layered media is not used.
In the Single Rate approach, the receiving capabilities of the users are not considered at all. Indeed, a conservative transmission policy is used that artificially keeps network utilization low. The Single Rate approach has the drawback that the media is usually sent at the lowest data rate even if all or most of the users could receive at higher data rates. The usage of higher data rates would result that a better quality version of the media could be sent during the same transmission time window available for this multicast media or network utilization could be improved.
The Replicated Rate technique can consider the receiving capabilities of the users. In this approach different quality versions of the media are transmitted at different data rates. Lower quality could be transmitted at lower data rates and higher quality could be transmitted at higher data rates. Because of this way of operation, however, this technique is resource consuming and bandwidth wasting, since it does not consider the layered structure of the media and the nested data rates of the network.
To sum up the problems, the existing solutions cannot provide differentiated quality to the users based on their receiving capabilities or require unnecessarily large amount of bandwidth. Today, the Layered approach can decrease the bandwidth usage only in the core networks (i.e. infrastructure networks); however, the bottleneck of the networks is in the wireless access part (i.e. the part that has direct communication with the user equipment).